1. Field of the Invention
The present invention relates to a static induction type semiconductor device which is capable of high-speed operation.
2. Description of the Prior Art
A static induction type semiconductor device has conventionally been used as one of the devices which control the conduction and cut-off of current. In a static induction type semiconductor device, there is a time delay in operation when conduction is established (hereinunder referred to as "turn on") or broken (hereinunder referred to as "turn off"). The speed-up of the turn-on and turn-off operations is therefore demanded.
An example of conventional static induction type semiconductor devices will now be explained with reference to FIG. 7.
This example is an n-channel static induction type thyristor, and FIG. 7 shows the sectional structure of two units thereof.
At the upper surface portion of a static induction type thyristor 100, cathode regions 10 consisting of an n-type semiconductor and a gate region 12 consisting of a p-type semiconductor are alternately provided. An anode region 14 consisting of a p-type semiconductor is provided over the entire surface of the lower surface portion of the static induction type thyristor 100. At the intermediate portion of the static induction type thyristor 100 between these upper and lower surface portions is provided a low impurity density region 16.
On the upper surface of the static induction type thyristor 100, first main electrodes 18 which are conducted to the cathode regions 10 and control electrodes 20 which are conducted to the gate regions 12 are provided. An insulator 22 is disposed between the gate region 12 and the first main electrode 18 so as to prevent the conduction therebetween. On the lower surface of the static induction type thyristor 100 is provided a second main electrode 24 which is conducted to the anode region 14.
This kind of the static induction type thyristor 100 can be used as a static induction type thyristor in which conduction is not established when the potentials of the control electrode 20 and the first main electrode 18 are maintained at the same level (hereinunder referred to as "normally-off static induction type thyristor). In this normally-off static induction type thyristor, a predetermined voltage is ordinarily applied between the first main electrode 18 and the second main electrode 24 so as to keep the anode region 14 in a predetermined high potential state with respect to the cathode region 10.
In the case of turn-on operation, the control electrode 20 applies a voltage to the gate region so that the gate region 12 has a positive potential with respect to the cathode region 10, whereby the static induction type thyristor 100 is turned on, and a current flows between the first main electrode 18 and the second main electrode 24.
More specifically, when the gate region 12 comes to have a positive potential, or a slight number of holes are supplied to a channel region 16a from the gate region 12, a large number of electrons are injected from the cathode region 10 to the low impurity density region 16 and further, a large number of holes are injected from the anode region 14 to the low impurity density region 16. Conduction is therefore established between the first main electrode 18 and the second electrode 24, in other words, the static induction type thyristor 100 is turned on. The channel region refers to a portion of the low impurity density region 16 which is below the cathode region 10 and between the gate regions 12.
In the case of cutting off the current which flows between the first main electrode 18 and the second main electrode 24, namely, in the case of turn-off operation, 0 V or a negative potential is applied to the gate region 12. The holes in the channel region 16a are drawn up to the gate region 12, and the injection of electrons from the cathode region 10 to the channel region 16a is stopped, whereby the current between the first main electrode 18 and the second main electrode 24 is cut off.
In the conventional static induction type thyristor 100, when it is in the conductive state, the holes exist at a high density over the entire range of the low impurity density range 16. All of these holes move toward the cathode region 10. Therefore, the density of the holes becomes high in the channel range 16a, namely, a portion of the low impurity density region 16 which is below the cathode region 10 and between the gate regions 12.
In the turn-off operation for cutting off the current between the first main electrode 18 and the second main electrode 24, the holes are drawn up to the gate regions 12. Since a large number of holes exist in the channel region 16a, the current is not cut off until these holes disappear, in other words, until all the holes are corrected by the gate regions 12, which fact makes high-speed cut-off difficult.
Incidentally, the time by which the operation is delayed until the holes disappear in the channel region 16a is referred to as storage time.
In order to cut off the current of such a conventional static induction type thyristor 100 at a high speed, the following measures, for example, have been proposed:
(a) that a life time killer such as gold is added when a substrate for a semiconductor is made; PA1 (b) that the life time of a semiconductor substrate is controlled by projecting electron beams or the like; and, PA1 (c) that the structure of the anode region 14 is improved.
However, according to the method of adding a life time killer such as gold and the method of controlling the life time by the projection of electron beams or the like, the number of the holes is reduced so as to enable the speed-up of cutting off operation, but the voltage drop in the conductive state increases, thereby disadvantageously increasing the leakage current in the cut-off state. On the other hand, according to the method of improving the structure of the anode region 16, the reduction in the number of the holes in the channel region 16a is too small to obtain sufficiently high-speed operation at the time of turn-off operation.